The American Civil War was the major war between the United States (The Union) and eleven Southern slave states which declared that they had a right to secession and formed the Confederate States of America, led by president Jefferson Davis.  The Union led by President Abraham Lincoln included all of the free states and four slave holding border states that were later joined by pro-Union counties of Virginia.  The Union opposed the expansion of slavery into territories owned by the United States and rejected any right of secession.
Fighting commenced on April 12, 1861, when Confederate forces attacked a United States (Federal) military installation at Fort Sumter in South Carolina, the first state to secede.

"During the first year, the Union assumed control of the border states and established a naval blockade as both sides raised large armies. In 1862 large, bloody battles such as Shiloh and Antietam were fought, causing massive casualties unprecedented in U.S. military history. A deadly combination of new weapons (including rifles using the Minié ball) and old battlefield tactics such as mass infantry charges led to thousands of casualties per major battle.
In September 1862, Lincoln's Emancipation Proclamation made the freeing of slaves in the South a war goal and gave a higher moral cause to the war, despite opposition from Northern Copperheads who tolerated both secession and slavery. The likelihood of intervention from Britain and France, both of which opposed slavery, was thus reduced. In addition, this policy allowed Union armies to liberate enslaved African-Americans, draining a valuable source of manpower from the South. The border states and War Democrats opposed emancipation at first, but gradually accepted it as part of total war needed to save the Union. European immigrants joined the Union Army in large numbers too. 23.4% of all Union soldiers were German-Americans; about 216,000 were born in Germany.
In the East, Confederate general Robert E. Lee assumed command of the Army of Northern Virginia and rolled up a series of victories over the Army of the Potomac, but his best general, Thomas Jonathan "Stonewall" Jackson, was killed at the Battle of Chancellorsville in May 1863. Lee's invasion of the North was repulsed at the Battle of Gettysburg in Pennsylvania in July 1863; he barely managed to escape back to Virginia with his badly mauled force. The Union Navy captured the port of New Orleans in 1862, and Ulysses S. Grant seized control of the Mississippi River by capturing Vicksburg, Mississippi in July 1863, thus splitting the Confederacy in two.
By 1864, long-term Union advantages in geography, manpower, industry, finance, political organization and transportation were overwhelming the Confederacy. Grant fought a number of bloody battles with Lee in Virginia in the summer of 1864. Lee's defensive tactics resulted in extremely high casualties for Grant's army, but Lee's army was shrinking daily due to casualties and desertions; he was forced to retreat into trenches around his capital, Richmond, Virginia.  Meanwhile, General William Sherman, the leader of the Union Military Division of the Mississippi, captured Atlanta, Georgia and began his March to the Sea, during which he destroyed a hundred-mile-wide swath of Georgia. In 1865, Confederate resistance collapsed after Lee surrendered to Grant at Appomattox Court House.
All slaves in the Confederacy were freed by the Emancipation Proclamation, which stipulated that slaves in Confederate-held areas, but not in border states or in Washington, D.C., were free. Slaves in the border states and Union-controlled parts of the South were freed by state action or by the Thirteenth Amendment, although slavery effectively ended in the U.S. in the spring of 1865."